There is known a grinding machine that increases the pressing force of centers to a work piece supported at both ends by the centers to synchronously drive both ends of the work piece for rotation with the friction forces of the centers to thereby grind the work piece as, for example, described in Patent Document 1. In the thus configured grinding machine, it is not necessary to chuck the end portions of a work piece or attach a drive fitting, so, for example, it is characterized in that the overall length of a cylindrical work piece may be ground without reclamping the cylindrical work piece, drive fittings, or the like, for various work pieces having different shapes may be not required, and various types of work pieces may be driven for rotation by only controlling the pressing force of the centers.
However, a work piece is driven by only the friction forces of the centers, so it is necessary to set sufficiently large pressing force of the centers in order to obtain friction forces such that a work piece does not slip because of grinding resistance. On the other hand, as the pressing force of the centers is excessively increased, a work piece warps to lead to a decrease in grinding accuracy, so there are technical restrictions that the pressing force of the centers cannot be blindly increased. Thus, depending on set center pressing force, the grinding resistance may be larger than the friction resistances of the centers and a slip may occur between the centers and a work piece to cause poor machining of the work piece.
Conventionally, there is, for example, known the technique for detecting a slip of a work piece as described in Patent Document 2 and Patent Document 3.